Technical Abstract:
This work evaluated ruminal protein degradation rates of legume hays that varied in tannin content. Two cuttings of 5 varieties of birdsfoot trefoil, (Lotus corniculatus), selected for different tannin contents but similar NDF and CP contents, and Spredor 4 alfalfa (control) were conserved as hay. Samples were ground (1 mm) and analyzed for chemical composition and tannin content. Protein degradation rate was determined using ruminal inocula in the Michaelis-Menten inhibitor in vitro method; 3 incubations were performed. Extent of degradation was estimated from net release of N in ammonia (phenol-hypochlorite colorimetry) plus amino acids and small peptides (o-phtalaldehyde colorimetry). Samples were also incubated in situ to estimate RDP. Data were analyzed using mixed procedure of SAS. There were differences among trefoils in tannin content with highest values for Dewey and Goldie and lowest for Exact. Within cut, alfalfa had the highest degradation rate and Exact had the numerically highest rate among trefoils. Rate was significantly affected (P < 0.001) by variety and cutting; there was also a variety*cutting interaction (P = 0.017). Regression between degradation rate and tannin content was: y = -0.0027 (±0.0007) x + 0.231 (±0.019) (r2 = 0.61). Regression between in situ RDP and tannin content was: y = -0.194 (±0.024) x + 84.0 (±0.68) (r2 = 0.87). Soluble N content must be determined to compute RDP within the in vitro method. Degradation rates were higher for the second-cut hays; this may be partly explained by greater NDIN content in first- vs. second-cut hays (21 vs. 18% of total N).